Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000
FXUS63 KDTX 190115
AFDDTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE DETROIT/PONTIAC MI
915 PM EDT THU SEP 18 2014

.UPDATE...

THE 00Z DTX SOUNDING SHOWED A SHARP INVERSION BASED NEAR 843MB.
A VERY SHALLOW LAYER OF MOISTURE WAS PRESENT UNDER THIS INVERSION.
OF NOTE HOWEVER WAS THE DEGREE OF DRY AIR ABOVE THE INVERSION.
ONGOING MID LEVEL SUBSIDENCE IS LOWERING THIS INVERSION AND
HELPING TO MIX SOME OF THIS DRY AIR INTO THE MOIST LAYER...WHICH
LED TO A RAPID CLEARING TREND OVER THE LAST COUPLE OF HOURS. A
SLIGHT UPSLOPE COMPONENT UNDER THE E-SE FLOW HAS HOWEVER HELD THE
CLOUDS IN PLACE OVER PORTIONS OF CENTRAL MI AND PORTIONS OF METRO
DETROIT AND THE IRISH HILLS. THERE REMAINS A POTENTIAL FOR
ADDITIONAL CLOUD DEVELOPMENT THROUGH NIGHT AS SOME RESIDUAL
MOISTURE MAY LINGER UNDER THE LOWERING INVERSION. RECENT TRENDS
HOWEVER SUGGEST A SLIGHT LOWERING OF FORECAST CLOUD COVERAGE
OVERNIGHT. CURRENT FCST MIN TEMPS STILL ARE REFLECTIVE OF A PERIOD
OF GOOD COOLING OVERNIGHT.

&&

.AVIATION...ISSUED 701 PM EDT THU SEP 18 2014

//DISCUSSION...

THE STRATO CU FIELD HAS BEEN SLOWLY ERODING THIS EVENING FROM BOTH
THE WANE IN THE DIURNAL CYCLE AND INCREASINGLY DIVERGENT LOW
LEVEL FLOW. THE HIGH TO THE NORTHEAST WILL CONTINUE TO NUDGE INTO
SE MI TONIGHT. THIS ALONG WITH STRONG DRYING AND SUBSIDENCE ATOP
THE MOIST LAYER WILL SUPPORT A CONTINUED CLEARING TREND. THE
PROSPECTS FOR COMPLETE CLEARING STILL LOOK IN QUESTION AS THE
STRENGTH OF THE LOW LEVEL INVERSION WILL ACT TO HOLD POCKETS OF
STRATUS OVER THE AREA WELL INTO THE NIGHT.

FOR DTW...SUBTLE SFC DRY AIR ADVECTION FROM THE NORTHEAST TONIGHT
AND THE POTENTIAL FOR LINGERING CLOUDS FROM TIME TO TIME SUGGESTS
REDUCTION IN VSBY FROM FOG WILL BE TOO LOW TO INCLUDE IN THE TAF
ATTM.

//DTW THRESHOLD THREATS...

* LOW CONFIDENCE IN CEILINGS AOB 5000 FT TONIGHT AND FRI MORNING.

&&

.PREV DISCUSSION...ISSUED 332 PM EDT THU SEP 18 2014

SHORT TERM...THROUGH TONIGHT

SURFACE COLD FRONT DROPPED DOWN THROUGH LOWER MICHIGAN EARLY THIS
MORNING...HELPED BY HIGH PRESSURE SETTLING INTO THE NORTHERN GREAT
LAKES. THIS IS EVIDENT BY NORTHEAST FLOW SHOWN ON SURFACE
OBSERVATIONS. MODELS INDICATE HOWEVER THAT THE ELEVATED PORTION OF
THE FRONT (925MB) IS LAGGING BEHIND THE SURFACE FEATURE...AND IS NOW
SITUATED FROM WESTERN WISCONSIN TO SOUTH-CENTRAL LOWER MICHIGAN TO
NEW YORK STATE. THIS LINES UP WELL WITH A STRATUS DECK SHOWN ON
SATELLITE IMAGERY. THIS UPPER FRONT WILL SLOWLY SAG SOUTHWARD
TONIGHT...AND BEND TO TAKE ON A MORE NORTHWEST TO SOUTHEAST
ORIENTATION BY LATE AS SURFACE HIGH PRESSURE SLIDES FROM EAST
OF LAKE SUPERIOR SOUTHEAST THROUGH QUEBEC. CLOUD FORECAST FOR
TONIGHT IS VERY TRICKY WITH THE BOUNDARY REMAINING OVERHEAD...AND AS
A SUBSIDENCE INVERSION STRENGTHENS UNDER HIGH PRESSURE. UNCERTAINTY
WITH THE CLOUD FORECAST THEN LEADS TO UNCERTAINTIES IN MIN TEMPS AND
FROST THREAT.

LIGHT NORTHEAST FLOW OFF LAKE HURON WILL WEAKEN THIS EVENING THEN
TURN MORE EASTERLY OVERNIGHT. MEANWHILE...ON THE WEST SIDE OF THE
STATE FLOW WILL BEGIN TO TURN MORE SOUTHWEST AS A SURFACE TROUGH
PROGRESSES THROUGH THE UPPER MISSISSIPPI RIVER VALLEY. BELIEVE WE
SHOULD INITIALLY SEE A DECREASE IN CLOUD COVER TONIGHT AS WE LOSE
DIURNAL HEATING...BEFORE INCREASING LOW-LEVEL CONVERGENCE OVER THE
CENTER OF THE STATE ALLOWS STRATUS TO FILL BACK IN UNDER THE
INVERSION OVERNIGHT. BEEFED UP CLOUD COVER OVER THE WESTERN HALF OF
THE FORECAST AREA...WHILE ALSO ADDING A MENTION OF FROST OVER THE
THUMB WHERE CLEARING WILL ALLOW MIN TEMPS TO DROP INTO THE MID 30S.
PATCHY FOG WILL ALSO BE POSSIBLE OFF LAKES ERIE AND ST CLAIR TOWARDS
SUNRISE AS LIGHT EASTERLY FLOW COMES OFF THE LAKE.

LONG TERM...

BURGEONING HEIGHT FALLS AS WE HEAD INTO THE WEEKEND...WITH MAX 500
MB HEIGHT FALLS OCCURRING SUNDAY MORNING AS A STRONG UPPER LEVEL
WAVE/TROUGH AXIS SWINGS THROUGH THE CENTRAL GREAT LAKES. AS THE
NORTHERN STREAM FLOW BEGINS TO BUCKLE OVER THE NORTHERN ROCKIES
TOMORROW...INCREASING SOUTH TO SOUTHWEST FLOW/WARM ADVECTION WILL
TAKE PLACE INTO SATURDAY MORNING AHEAD OF A COLD FRONT. MODEL
DISCREPANCIES DEAL WITH THE UPPER LEVEL LOW/TROUGH ALONG THE WEST
COAST AND THE AMOUNT OF UPPER LEVEL ENERGY WHICH SCOOTS OFF TO THE
NORTHEAST...AS THE LOW CLOSES OFF AND LOSES THE LINKAGE WITH THE
STRONG WESTERLIES. THIS IMPACTS THE AMOUNT OF LOW LEVEL MOISTURE
WHICH ADVECTS UP FROM TEXAS/NEW MEXICO. THIS IS IMPORTANT AS AIRMASS
WILL INITIALLY BE EXTREMELY DRY TOMORROW OVER SOUTHEAST
MICHIGAN...AS 700 MB DEW PT DEPRESSIONS RESIDE IN THE 30 TO 50
DEGREE RANGE. THERE IS MODEST CONCERN SURFACE/NEAR SURFACE
FOG/CLOUDS COULD BECOME TRAPPED UNDER DEVELOPING INVERSION
TOMORROW...AS WISCONSIN IS DEEPLY SOCKED IN TODAY...BUT THAT WAS
STRONGLY INFLUENCED BY THE FLOW OFF LAKE MICHIGAN THIS MORNING. AS
ELEVATED WARM AIR (8C AT 700 MB) STREAMS IN FRIDAY NIGHT-SATURDAY
MORNING...MID LEVEL LAPSE RATES WILL BE IMPRESSIVE (7+ C AT 700-500
MB)...BUT THAT COLUMN WILL BE DRY...AND AS MOISTURE AXIS ARRIVES
SATURDAY AFTERNOON/EARLY EVENING...MOISTENING UP OF THE COLUMN WILL
DWINDLE THE LAPSE RATES TO MORE MANAGEABLE 6.0-6.5 C/KM. MOISTURE
FEED IS NARROW...BUT SOLID WITH 700 MB DEW PTS IN THE 3-5 C
RANGE...WITH PW VALUES JUST UNDER 1.75 INCHES...PER BLEND
NAM/GEM/GFS. STILL...NOT TOTALLY CONVINCED ON COVERAGE/DEVELOPMENT
DUE TO LIMITED INSTABILITY...AS MUCAPES REMAIN BELOW 1000 J/KG...IF
NOT UNDER 500 J/KG. CONCERN IS EVEN WITH MEAGER INSTABILITY...STRONG
WIND FIELDS...45-50 KNOTS AT 850 MB DRAWS CONCERN FOR STRONG WINDS
OF 45-60 MPH WITH ANY STRONGER CONVECTIVE ACTIVITY...BUT NEAR
SURFACE INVERSION STILL WILL PROBABLY BE TOUGH TO OVERCOME TO GET
WINDS TO THE GROUND...ESPECIALLY IF THE FRONT IS SLOWER...POST
DAYLIGHT HOURS.

MORE VIGOROUS HEIGHT FALLS WILL OCCUR FOR THE SECOND HALF OF THE
WEEKEND AS STRONG UPPER WAVE EJECTS OUT OF THE GULF OF ALASKA. A
STRONG MID LEVEL DRY SLOT WILL DEVELOP AND TRACK THROUGH SUNDAY
MORNING BEFORE SECONDARY COLD FRONT AND COLD ADVECTION SETS IN
DURING THE DAY...LIKELY LEADING TO SCATTERED SHOWERS WITH THE STEEP
LOW LEVEL LAPSE RATES...WITH AN ISOLATED THUNDERSTORM NOT OUT OF THE
QUESTION AS SOUNDINGS DO SHOW EQL LEVELS UP AROUND -20 C ISOTHERM.

AS THE UPPER WAVE MOVES EAST OF THE REGION...HIGH PRESSURE WILL FILL
IN FOR SEVERAL DAYS. THIS WILL KEEP ALL WEATHER HIGHLIGHTS AWAY FROM
THE CWA FOR THE EXTENDED PERIOD. AS WE NEAR CLOSER TO EARLY NEXT
WEEK...EXPECT A FEW MENTIONS OF AM FOG. TEMPERATURES WILL GRADUALLY
BE ON THE INCREASE THROUGH THE WEEK AS WELL WITH 500MB HEIGHTS
SLOWLY RISING FROM 570 TO 586 DAM.

MARINE...

SURFACE HIGH SETTLED OVER THE CENTRAL GREAT LAKES TONIGHT WILL LEAD
TO LIGHT WINDS. SOUTHERLY FLOW WILL BEGIN TO INCREASE TOMORROW...AND
ESPECIALLY TOMORROW NIGHT AHEAD OF A COLD FRONT...WHICH WILL TRACK
THROUGH SATURDAY NIGHT. ALTHOUGH WARM AIR WILL STREAM INTO THE GREAT
LAKES REGION...LEADING TO MORE STABLE LOW LEVEL PROFILES...SMALL
CRAFT ADVISORIES STILL MAY BE NEEDED OVER THE WEEKEND...ESPECIALLY
OVER SAGINAW BAY. DUE TO THIS WARM AIR...WINDS ARE EXPECTED TO
HOLD IN THE 25 TO 30 KNOT RANGE OVER THE OPEN WATERS OF LAKE HURON.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LAKE HURON...NONE.
LAKE ST CLAIR...NONE.
MI WATERS OF LAKE ERIE...NONE.

&&

$$

UPDATE.......SC
AVIATION.....SC
SHORT TERM...HLO
LONG TERM....SF/MM
MARINE.......SF


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